Is this a Good Day to go Flying? By James Goss      How do you determine if it is a good day to go to the flying field? Do you use one of these top ten conditional values to help make that decision? It's a nice sunny day and you think you will drive on out to the field just to see what's going on. You haven't got anything else that is worthwhile to do. You have a new plane and you are going to see it fly today regardless of the weather. Your wife runs you out of the house and you haven't got any other place to go. You are going to set and talk all day at the field so the weather doesn't matter. You are actually going to the field to fly a plane. You are going to watch other pilots fly so you will not be rebuilding your plane next week. I need to stay home and build today instead of going to the field. I can't go flying today because I forgot to charge last night. There are probably too many people at the field for me to fly today.      Actually these are some pretty good reasons for going or not going to the field, I have used all of them at one time or another. Here is one that I bet you have never heard before; “The air is not dense enough for me to fly today”. This could be a valid point because the density of the air really does affect the way our planes fly. To find the density of anything, you divide its mass by the volume it occupies. Air density is usually measured in kilograms per cubic meter. At sea level, one cubic meter of air will weigh 1.275 kilograms if the air is dry and at 0 degrees Celsius. The density of air depends on three things: temperature, pressure, and moisture. It is easy to see why temperature is involved because we all know that if air is heated it will expand. If the air expands there are fewer air molecules in that same cubic meter of space. Fewer molecules means less weight and less weight means a lower density. So on a really hot day the air density will be less, assuming that pressure and moisture is fixed.      The air pressure takes us in the opposite direction from that of temperature. If the pressure goes up the air will be compressed into a smaller volume. Now we can get more air molecules into the same one cubic meter of space. This increases the weight and also the density. As you know altitude can change the air pressure. The higher you go the less pressure you have per unit area. The air's density is highest at the lower elevations where the pressure is high and the temperature is low. Here in Alabama the highest point is on Cheaha Mountain, it is 2405 feet above sea level. If you were flying atop this mountain, your plane would not have as much lift and your prop would not develop as much thrust. Our flying fields are in a pretty good location, as far as air pressure goes. Starting at Childersburg, the elevation is 419 feet, Cropwell is 478 feet, Talladega is 555 feet, Oxford is 658 feet and Jacksonville is 672 feet above sea level.      Most people will not believe that humid air is less dense than dry air. In the early 1800s, Avogadro discovered that a fixed volume of gas, at the same temperature and pressure, would always have the same number of molecules no matter what gas is in the container. This means that if a container is filled with gas and some lighter molecules goes into the container, some of the heavier molecules in the container will be forced out.  This will decrease the density of the gas that remains in the container. This is what happens when water vapor is added to the air. If you recall, air is made up of several gasses such as nitrogen, oxygen and other elements, but mostly nitrogen. Water is made up of hydrogen and oxygen. The atomic weight for water is less than that for the gasses found in air. Remember that we are talking about moisture in the form of gas, not water, of course water is heavier than air. So as moisture is added to the air, the air becomes lighter in weight and has less mass per unit volume.      If the air is denser our planes will have a harder time pushing the molecules to the side as the plane moves through the air. We call this resistance that the air offers to the plane, drag. Drag increases with air density and will slow an object down as it passes through the air. An example of this is in the game of baseball. A homerun will travel farther if hit in a high altitude environment as compared to a low altitude more dense air environment. With our model airplanes it is a little different. With less dense air, an airplane will not obtain the same lift from its wings as it would on a more dense air day.      The thrust that the propeller develops will also be decreased on a less dense air day. If it is a humid day the prop can't grip the air as good and your engine will not pull the plane through the air as well. The effect on thrust and lift is greater than the effect caused by the drag. So even though there is less drag on a humid day, the pounds of force developed by the prop is reduced to a value that still slows the airplane down. And finally the engine is not going to produce as much power on a humid day because the total volume of usable air that passes through the carburetor is reduced. By the air being less dense, there is less oxygen available for the engine to operate at its most efficient operating point.      The ideal day to fly then would be on a cool and dry day with little or no wind. We would get increased lift from the wing and increased thrust from the prop that overrides the increased drag. I am sure that you have noticed your plane flying better on some days that on others. Just remember that air density has a lot to do with how well your plane will fly. It may not always be the fuel you are using that gives you a poor day at the field.